The invention relates to a method for operating an internal combustion engine, especially of a motor vehicle. In the method, fuel is injected directly into a combustion chamber either in a first operating mode during a compression phase or in a second operating mode during an induction phase. In the method, a switchover is made between the two operating modes and the injection duration, which influences the injected fuel mass, is controlled (open loop and/or closed loop) differently in the two modes of operation. Furthermore, the invention relates to an internal combustion engine, especially for a motor vehicle, having an injection valve with which fuel can be injected directly into a combustion chamber either in a first mode of operation during a compression phase or in a second mode of operation during an induction phase. The engine has a control apparatus for switching between the two operating modes and for controlling (open loop and/or closed loop) differently the injection duration in the two modes of operation. The injection duration influences the injected fuel mass.
Systems of this kind for direct injection of fuel into the combustion chamber of an internal combustion engine are generally known. A so-called stratified charge operation as first operating mode and a so-called homogeneous operation as second operating mode are distinguished. The stratified charge operation is used especially for smaller loads; whereas, the homogeneous operation is used for larger loads applied to the engine.
In stratified charge operation, the fuel is injected into the combustion chamber during the compression phase of the engine in such a manner that, at the time point of the ignition, a fuel cloud is located in the direct vicinity of the spark plug. This injection can take place in different ways. Thus, it is possible that the injected fuel cloud is already located at the spark plug during or directly after the injection and is ignited by the spark plug. Likewise, it is possible that the injected fuel cloud is conducted to the spark plug via a charge movement and is only then ignited. In both combustion methods, no uniform fuel distribution is present, instead, a stratified charge is present.
The advantage of the stratified charge operation is that there, with a very small fuel quantity, the applied smaller loads can be taken care of by the engine. Larger loads can, however, not be satisfied with the stratified operation.
In homogeneous operation, which is provided for such larger loads, the fuel is injected during the induction phase of the engine so that a swirling and therefore a distribution of the fuel can still easily take place in the combustion chamber. To this extent, the homogeneous operation corresponds approximately to the operation of internal combustion engines wherein fuel is injected into the intake manifold in the conventional manner. As required, the homogeneous operation can be used also for smaller loads.
In stratified charge operation, the throttle flap in the intake manifold, which leads to the combustion chamber, is opened wide and the combustion is essentially controlled (open loop and/or closed loop) only by the fuel mass to be injected. In homogeneous operation, the throttle flap is opened or closed in dependence upon the requested torque and the fuel mass, which is to be injected, is controlled (open loop and/or closed loop) in dependence upon the inducted air mass.
In both operating modes, that is, in the stratified charge operation and the homogeneous operation, the fuel mass, which is to be injected, is additionally controlled (open loop and/or closed loop) in dependence upon a plurality of additional operating variables with a view to an optimal value with respect to fuel saving, exhaust-gas reduction and the like. The control (open loop and/or closed loop) is then different in the two operating modes.
In the control (open loop and/or closed loop) of direct-injecting internal combustion engines, each of the two operating modes should be considered separately. Likewise, it should be guaranteed that, for the switchover especially from the homogeneous operation into the stratified charge operation, the torque, which is outputted by the engine, remains constant.
The object of the invention is to provide a method for operating an internal combustion engine which makes possible a constant torque switchover from the first mode of operation into the second mode of operation.
This object is solved in a method of the above kind in accordance with the invention in that the injection duration is influenced by a function when there is the switchover from the first operating mode into the second operating mode. In an engine of the type mentioned initially herein, the object of the invention is solved in that the injection duration can be influenced by a function via the control apparatus with the switchover from the first operating mode into the second operating mode.
In this way, it is possible to compensate the transient performance of the intake manifold pressure of the engine caused by the switchover from the stratified charge operation into the homogeneous operation. With the function, the injection duration can be changed in such a manner that the above-mentioned transient performance is compensated. With the invention, it is achieved that the outputted torque of the engine remains essentially constant when switching over.
It is especially advantageous when the function is determined from a simulation computation or model computation. In this way, a torque, which is essentially constant, is obtained with the switchover from the stratified charge operation into the homogeneous operation.
In an especially advantageous embodiment of the invention, the function is changed in dependence upon one or several operating variables of the engine. In this way, the constancy of the outputted torque can be further improved with the switchover from the stratified charge operation into the homogeneous operation.
It is advantageous when the function runs from a positive start value, which is greater than the value xe2x80x9c1xe2x80x9d to an end value xe2x80x9c1xe2x80x9d. In this case, it is purposeful when the function is multiplicatively coupled to an injection duration provided for the second mode of operation.
In an advantageous embodiment of the invention, the function is pregiven in a time region wherein an increase of the injection duration, which is really provided for the second operating mode, is required in order to hold the outputted torque of the engine constant. The function is therefore only effective in that time region wherein a correction or compensation of the torque, which is outputted by the engine, is required. Otherwise, the function does not become effective.
The realization of the method of the invention in the form of a control element is of special significance. The control element is provided for a control apparatus of the engine, especially of a motor vehicle. A program is stored on the control element which can be run on a control apparatus, especially on a microprocessor, and is suitable for carrying out the method of the invention. In this case, the invention is therefore realized by a program which is stored on the control element so that this control element, which is provided with the program, defines the invention in the same way as the method for whose execution the program is suitable. As a control element, especially an electric storage medium can be used, such as a read-only-memory.